1. Field of the Invention
The present invention relates to a light guide plate; more particularly, to a light guide plate for use with an edge type back light module of a liquid crystal display (LCD), and an LCD comprising the light guide plate.
2. Descriptions of the Related Art
Liquid crystal displays (LCDs) are mainstream products on the display market. Not only does it save power and emit low radiation, it is also lightweight and portable. It is widely used on appliances such as televisions, general monitors, laptops, global position systems (GPS), and mobile communicating devices. That being said, conventional monitors are gradually being replaced by LCDs.
The backlight module is one of the key components for providing uniform and sufficient lights for an LCD panel. A conductive type backlight module primarily comprises a lamp, lens sheet (or prism sheet), light guide plate, diffusion sheet, reflect sheet, protective film, and optical film.
Backlight modules are divided into two categories, namely, direct types and edge types. An illumination device for use with the edge type backlight module is disposed on the outer edge of the light guide plate. A reflector is disposed under the light guide plate for gathering and reflecting lights through the panel. In comparison, the edge type backlight module has the benefit of being slimmer than the direct type. Moreover, the edge type also has fewer heat problems.
A backlight module demands uniform and high brightness levels, low cost, and light weight. Thus, the market is apt to employ the edge type backlight module because certain dimensions of LCDs comprising the edge type module have those advantages. However, a conventional light guide plate that forms uniform V-shape grooves for guiding lights to a light output plane tends to cause dark areas adjacent to the light input plane. The uniformity of the overall brightness is, therefore, hard to control.
FIG. 1A is a conventional light guide plate that has uniform V-shape grooves (i.e. the top angles of the V-shape grooves are equivalent between the two opposing end portions of the light guide plate, i.e. y1=y20). FIG. 1B is a schematic view illustrating the dark areas adjacent to the light input portion in view of FIG. 1A. An illumination device 1 (preferably a tube lamp) emits lights into the light guide plate 2 through a first end portion 24 of the light guide plate 2. Initially, the lights in the light guide plate 2 usually travel in a total-reflections manner, following Snell's Law. Soon after, the total-reflections gradually diminish and the lights continuously travel in reflection and refraction paths, wherein the refractive lights emit from a top surface 22 as visible lights. Accordingly, the more frequently the total reflections occur in the light guide plate 2, or the more delay time between reflection and refraction paths, the less frequently the refractive lights will emit from the effective displaying area. It follows that the dark areas will increase and the high performance visible region will decrease, which means that the brightness of the overall display is not uniform. FIG. 1B shows the dark areas occurring at the light input portions according to the above-mentioned problems. These problems negatively affect the products' performances in brightness and quality.
To diminish total reflections early on when the lights travel in the light guide plate, a design that modifies this light guide plate (as shown in FIG. 2) so that it has a tapered shape (as shown in FIG. 3) is further created to initially introduce the lights refracting out of the light guide plate, thereby reducing the undesired dark areas adjacent to the illumination device 1.
Other similar designs have been developed as well. For example, U.S. Pat. No. 5,485,354 discloses a light guide plate that has a uniform thickness and a plurality of V-shape grooves formed thereon with equivalent top angles. The V-shape grooves are formed parallel to the light input plane (which is adjacent to the tube lamp). The light guide plate, therefore, transmits the lights from the tube lamp to the light output plane. Another example, U.S. Pat. No. 6,576,887, discloses a gradually tapered light guide plate that has a plurality of trapezoidal grooves formed parallel to the light input plane (which is adjacent to a tube lamp). Likewise, the light guide plate transmits the lights from the tube lamp to the output plane.
However, too many total-reflections still exist in the light paths that run within the light guide plate in the mentioned conventional designs. These problems do not match the needs of a high quality product that demands uniform brightness in the overall display area.
Thus, a novel structure that further reduces the dark areas on the edges, widens the viewing angles, and improves the overall uniformity in brightness, is required in this field.